Electrical resistance-type heater



June 12, 19 2 w. M. FORD 3,03 0

ELECTRICAL RESISTANCE-TYPE HEATER Original Filed July 6, 1959 4Sheets-Sheet 1 INVENTOR. WILL/AM FORD June 12, 1962 W. M. FORDELECTRICAL RESISTANCE-TYPE HEATER Original Filed July 6, 1959 4Sheets-Sheet 2 IN V EN TOR.

WILL/AM M. F0120 June 12, 196 w. M. FORD 3,039,071

ELECTRICAL RESISTANCE-TYPE HEATER Original Filed July 6, 1959 4Sheets-Sheet 3 IN VEN TOR.

WILLIAM M. FORD June 12, 1962 w. M. FORD ELECTRICAL RESISTANCE-TYPEHEATER 4 Sheets-Sheet 4 Original Filed July 6, 1959 R M V w. m i 1 M m 0WM MA 9 8 (1 u e n A r o C BY WILLIAM M. FORD m fli ATTORNEY UnitedStates Patent 6 Claims. (Cl. 333-294) This invention relates to anelectrical resistance-type heater used in apparatus for growing largecrystals by a modified Czochralski technique. This application is adivision of application Serial Number 824,975, filed July 6, 1959.

Heretofore, the Czochralski technique gave satisfactory results forgrowing crystals up to about two inches in diameter and, with somemodifications, up to about five inches in diameter. The presentinvention makes it possible to grow crystals from six to twenty inchesin diameter and larger.

For those unfamiliar with the Czochralski technique, it can be brieflysummarized. A seed crystal, attached to one end of a rod, is dipped intoa crucible of molten material and is then retracted slowly, pulling someof the material up with it. The crucible is heated warmer than theatmosphere above it, which is below the freezing or solidification pointof the molten material. :In fact, the sides and bottom of the crucibleare, when pulling begins, above the melting point of the material, andthe center of the upper surface of the molten material is between themelting point and the freezing point. As a result the material that ispulled'up crystallizes and pulls up the material below it, which alsocrystallizes in turn. The process is continued until all the materialhas been crystallized in this manner. An important fact is that thiskind of crystallization often produces a single large crystal and, ifproperly done, always produces crystals of single-crystal density. I

For small crystals, the technique is not difiicult to practice. Specialfurnaces, with thermocouples or devices sensitive to infrared radiationand recorder and control devices for the crucible heaters, have enabledits use for such difiicult materials as silicon and for crystalssomewhat larger than two inches. But heretofore, the crystal size hasbeen limited to a maximum of less than six inches in diameter and eventhen the formation of the larger crystals has been difi'icult andexpensive.

The invention is applicable to many, if not all, types of crystals.Large crystals of silicon, germanium, aluminum, iron, magnesium,molybdenum, gallium arsenide, gallium antimonide, magnesium dioxide,calcium fluoride, lead selenide, indium antimonide, indium arsenide,aluminurn phosphide, and aluminum-antimonide can be made by thisinvention, but so can others. No restriction of the invention isintended by using in the following discussion, silicon as an example.Silicon is one of the'most useful crystal materials, one of the hardestto make-in large sizes, and one on which much work. has been done overmany years without being able to get large, crystals. In

addition to its infrared uses, silicon (and some other materials)are-useful as semiconductors, for solar batteries, photovoltaicdetectors, photoconductive detectors, transistors, rectifiers, andPeltier-junction materials, The present invention makes large crystalsof silicon not only possible but also feasible and economical.

The present invention provides a novel electrical control of the thermalgradient in the molten material regulating the liquid-solid stateequilibrium, and thereby makes possible the manufacture of singlecrystalline and dense polycrystalline material of diameters limited onlyby the power supply and the size of the crucible.

Other problems, as well as other objects andadvantages ice of theinvention, are dealt with later on in this specification or will bequite apparent from a careful reading thereof.

In the drawings:

FIG. 1 is a view in front elevation of a furnace assembly embodying theprinciples of the invention.

FIG. 2 is an enlarged view in elevation and in section A of the furnaceof FIG. 1.

FIG. 3 is a bottom plan view in section taken along the line 3--3 inFIG. 2.

FIG. 4 is a view in elevation and in section of a novel contact assemblythat supports and supplies electrical energy to the heaters.

FIG- 5 is an electrical circuit diagram of the furnace heaters and theircontrol devices.

As shown in FIG. 1, a furnace 25 embodying the principles of thisinvention may be supported by a liquid cooled plate 26 on standards 27above a base 28. A seed rod 30 is mounted axially with respect to thefurnace 25 and enters it from above. A motor 31 may be supported on thebase 23 for raising and lowering the seed rod 30 through gears 32, shaft33, gear boxes 34, rotating worms 35, and geared blocks 36. The blocks36 carry a plate 37 that, in addition to serving as a bearinged supportfor the seed rod 30, also supports a motor 38 that rotates the seed rod30. A similar motor 39, carried by the furnace 25 below the plate 26,rotates a crucible inside the furnace 25.

The furnace 25 (see also FIG. 2) has a metal shell or housing 40enclosing a chamber 41. The shell 40 is made from metal capable ofwithstanding high temperatures and is cooled exteriorly, both by air andby cold water flowing through a coil '42. The chamber 41 is gastight,the shell 40 being sealed to the bottom plate 26 and to a top plate orcover 43, all openings through the plates 26 and 43 being tightlysealed. The plates 26 and 43 are preferably water cooled. All air isflushed from the chamber 41, and it is filled with an inert atmosphere,such as argon, through a port 44.

The top plate or cover 43 has a central sealed opening 45, through whichthe seed rod 30 passes, being both rotatable and reciprocable withrespect thereto and sealed. Windows 46 enable viewing and a prism 47enables measurement of the diameter of the crystal being grown. Ports 48lead to the water-cooling chamber inside the cover 43. V

A very important part of this invention is the structure 1 of theelectrical-resistance type heating elements. These comprise a peripheralheater 50 and a separate bottom heater 51, and the peripheral heater 50constitutes a plurality of segments capable of separate adjustment.

In a silicon furnace 25 both the heaters 50 and 51 are preferablyconstructed of graphite. Molybdenum disili- 'cide and other suitablematerials may be used if desired.

Graphite has a very low vapor pressure at the high melting. temperatureof silicon, and it has the necessary conductivity-resistancecharacteristics lacking in most other materials that do not vaporize athigh temperatures. Thus it does not tend to adversely affect the purityof the silicon, as do materials that produce contaminating vapors at theoperating temperature of the furnace, e.g., above 1420 C. for a siliconfurnace. V

In essence, the heater 50 may be considered as a graphite tube 52 with aradially inset portion 53 and a shoulder 54 at its lower end. Notches 55extend from the bottom to near the top where they end at bridges 56.Notches 57 extend from the top to the bottom, except for bridges 58 atthe inner periphery of the portion 53. Thus a complete circuit ofgraphite bar is described, the graphite havingza substantially constantcross-sectional area throughout, to give a constant resistance throughany section;

At the bottom and substantially coplanar with the shoulder 54- is theannular bottom heater 51. Notches 60 extend radially from the innerperiphery 61 to outer peripheral bridges 62, while notches 63 extend inradially from the outerperiphery 64 to inner peripheral bridges 65.Again there is structure providing a constant-cross sectional-area pathof graphite around 360, so that the heater 51 will heat evenly.

As shown in FIGS. 1 and 2 a pedestal housing 70 depends from the plate26. The motor 39 drives a worm gear71 .which drives an annular piniongear 72. The gear 72 may rest on a suitable bearing 73 which, in turn,may rest on a shoulder 74 of the housing 70, and a hollow graphitepedestal 75 extends upwardly therefrom into the chamber 4-1 and throughthe opening 61 in the heater 51. Since the upper end of the pedestal 75is very hot, the housing 7 is preferably water cooled.

At the upper end of the pedestal 75 is threaded an annular graphiteflange 76, On this sits a cup-shaped graphite susceptor 77, held inplace by an integral depending ring 78 on its lower surface. Inside thesusceptor 77 is a crucible 80 of interiorly glazed quartz. I havediscovered that transparent fused quartz is not necessary; opaquequartz, if interiorly glazed, gives as good results and costs only afraction as much. This discovery is quite important, for it had beenthought heretofore that opaque quartz crucibles could not be used inmaking silicon crystals, as indeed they cannot unless the interiorsurface is well self-glazed. But once so glazed, there is as littletendency of the opaque quartz to combine with the silicon and formsilicon monoxide as with the fused transparent quartz. Since thecrucibles are almost invariably broken, and since opaque quartz costsless than one-tenth as much as transparent quartz, the economics 'of theprocess is considerably alfected by this discovery.

Moreover, transparent quartz is not commercially available in sizesgreater than about 6 inches in diameter, whereas opaque quartz isavailable from stock up to 48 inches in diameter.

As will be apparent from the drawings and the foregoing description, themotor 39, through the gears 71 and 72, drives the pedestal 75 andtherefore rotates the susceptor 77 and crucible 80 together. Therotation is slow (usually about 3-20 r.p.m.), serving principally tohelp avoid the formation of uneven temperature zones. By itself,rotation is not sufficient, however, for reasons that will soon becomeapparent.

At the bottom of the hollow pedestal 75 is a thermopile 81, such as aRayo tube. This tube looks through the tubular passage 82 in thepedestal 75 up to a spot 83 at the center of the bottom of the susceptor77 and detects small changes in temperature and then actuates a controldevice to take corrective steps. The arrangement is conventional and sowill be described only briefly. The thermopile 81 (see FIG. is connectedto a DC. amplifier 84 through a bucking circuit 85, the bucking circuit85 having a battery 86, a load resistance 87 and a manually setpotentiometer 88. Thus, once the potentiometer 88 is set, departuresfrom the corresponding temperature on the spot 83 cause electric currentto be applied to the amplifier 84. This amplified difference then goesto a suitable control unit that takes corrective action, such as a Leedsand Northrup Speedomax H recorder-controller 90, which is connected intothe heating circuit, as by a lead 91.

Thus, the control device 90 keeps the susceptor spot 83 at a temperatureset by the potentiometer 88. This temperature level may be raised andlowered by adjusting the potentiometer 88, the control 99 acting both onthe peripheral heater 50 and the bottom heater 51.

The side wall 40 of the furnace is shielded from the heaters by atubular shield 92 of opaque quartz, which is glazed on its interiorsurface and reflects the heat back to the heaters 50 and 51. Itsreflectiveness is afiected,

however, during operation by small condensations of tend to result inletting one segment of the heater 50 get cooler than the other segments.This problem, and others causing cool areas on the peripheral heater 50,are solved by the present invention, in which the unitary heater 50 isdivided electrically into at least three arcuate cylindrical segments,four such segments 93, 94, 95, and 96 being shown in the deviceillustrated here. Each of these segments can be separately controlled,and all of them are also controlled together by the aforementionedcontrol system.

As shown in FIG. 5, each segment 93, 94, 95, and 96 and the bottomheater 51 is located across a respective secondary 97 of a separatetransformer 98, there being five transformers 98or as many as there areheating units. The primary 99 of each transformer 98 is separatelycoupled through a saturable reactor 100 to a source of primary power bylines 101 and 162. Typically this source supplies 220 volts, singlephase at 600 amperes. The saturation coil of each reactor 100 isenergized by the output of a magnetic amplifier 103. Each magneticamplifier 103 is connected to a rheostat 104, and all the rheostats 104are connected to the control line 91.

Thus, by setting each rheostat 104 separately, the segments 93, 94, 95,and 96 of the heater 50 can be adjusted relatively to each other and tothe heater 51. Then, the potentiometer 88 can beused to raise and lowerthe current to all the heaters and their segments while leaving theseparate relative adjustments undisturbed. This control is what enablesme to grow large crystals where others had failed.

An important factor in the present invention is that the necessaryseparate adjustment of peripheral segments, which is essential togrowing large crystals, cannot be obtained from radio-frequencyinduction-coil furnaces. There, the annular passage of each turn of thecoil prevents such adjustment. The prior-art furnaces all use inductioncoils, which are incapable of the necessary delicate adjustmentsachieved by this invention. The use of electrical-resistance typeheaters is therefore an important feature of the invention. Notehowever, that segmentation of the heater 50 does not mean four separatelengths of graphite. All it requires is four terminals 105, coupled tothe transformers 98 with the proper phase balance, as shown in FIG. 5,with alternate secondaries 97 out of phase,

The prior art had always turned away from high-heat furnaces usingresistance heaters, because ordinary resistance heaters cannot transmitenough power attemperatures of 1500" C., as is required with silicon andas my graphite heaters 50 and 51 can, and because of the difiiculty ofconnecting power to such heaters. This invention has also solved thatdifiicult problem.

The problem arises for several reasons: (1) copper cables cannot beconnected directly to the heaters 50, 51 for the copper would melt; (2)graphite and metal have greatly different temperature coeflicients ofexpansion and this tends to cause either element breakage orloosecontacts, with resultant erratic behavior; (3) graphite-tometalcontacts have a high surface resistance, especially at low contactpressures; (4) copper oxidizes rapidly at high temperatures, resultingin high contact resistances; (5) hot graphite oxidizes rapidly in air;and (6) the high heat concerned is diflicult to dissipate. a

The invention solves this by (1) making graphite-tographite contact atthe heaters, so that there is no problem of melting, uneven expansion,loose contacts, or surface resistance; (2) providing graphite to copper(silicon bronze) contact over extended areas and in such a way that thegraphite is hot and the copper cool, thereby creating hi'gh contactpressures that reduce the surface resistance; (3) cooling this contactarea at a point outside the furnace; and (4) keeping all the graphiteand the graphiteto-copper contact inside an inert gas atmosphere, whereoxidation cannot take place.

In the present invention the heater 50 has four graphite terminals 105with sockets 106 supported by four contact assemblies 110 and the heater51 has two stepped graphite terminals 107 with sockets 108 supported bytwo contact assemblies 111. Except for minor geometry and sizeconsiderations, these assemblies 110 and 111 are identical; so thefollowing description of an assembly 110 applies to the assembly 111 aswell. Reference will be mainly to FIG. 4.

The assembly 110 is made up of a graphite post 112, a silicon-bronzescrew 113, and a silicon-bronze receptacle 114.

The post 112 is solid graphite. It includes a thick column 115 that hasa shoulder 116. The shoulder rests on a support-insulator ring 117,preferably of Transite (asbestos) that in turn rests in an annularrecess 119 in the plate 26 and insulates the post 112 from the plate 26,while centering it in an opening 129 through the plate 26. An upper endportion 118 of the graphite post 112 threads into the heater socket 105and provides the graphite-to-graphite contact :at that point. Aninteriorly and exteriorly threaded portion 120 of the post 112 extendsdown below the shoulder 116 and through and below the plate 26, and intoit is threaded the silicon-bronze screw 113 to provide considerablecontact area, only a head 121 projecting out at the bottom.

The silicon-bronze receptacle 114 has an interiorly threaded passage 121which is threaded around the portion 120 of the graphite post 112 toprovide considerably more contact area and enclosure of the graphite andof the graphite-to-copper contact. Its upper end 122 is provided with anannular groove 123 in which a sealing and insulating O-ring 124 (it maybe made from silicone rubber) is seated. The O-ring 124 bears tightlyagainst the bottom of the plate 26 and insulates the receptacle 114 fromit and also prevents air from reaching the graphite post 112. For oneproblem was that graphite oxidizes readily and to give it access to airat the high temperatures involved, would mean rapid consumption of thepost 112. This structure prevents that by encasing it in thesiliconbronze receptacle 114, where the post 112 has access only to theinert atmosphere of the chamber 41.

Cooling is obtained by an aluminum tube 125 which surrounds thereceptacle i114. and provides a chamber 126 closed by O-rings 127 exceptfor ports 128 through which cooling water enters, circulating aroundalmost the whole length of the receptacle 114 and the depending portion120 of the post 112. By cooling the silicon bronze receptacle 114, thehot graphite 120 exerts considerable contact pressure on the receptaclethreads 121 increasing conductivity.

The bottom of the receptacle 114 is provided with a threaded bindingpost 130 to which the electrical cables are attached. Contact is made byengagement of the silicon-bronze screw 113 interiorly as well as by theexterior sleeve portion 121 of the receptacle 114. The screw head 121fits snugly in a socket 131 to give current transfer Without thepossibility of exposing any graphite to air.

Operation begins'by placing silicon chunks in the crucible 80, insertingthe crucible 801into1the susceptor '77 and in the furnace 25, closingthe top 43 tightly, and putting an inert atmosphere under pressure intothe chamber 41 through the ports 44. Then, maximum power is applied toall the heater segments 93, 94, 95, and 96 and to the heater 51 whilealso rotating the crucible 80, by the motor 39 rotating the pedestal 75.The seed rod 30 is kept up high at this stage or even outside thechamber 41 to protect the seed. The chamber 41 can be kept closed exceptduring insertion of the seed rod 30; external gases do not enter theatmosphere within the chamber 41, during insertion, since the gas insidetentiometer 88. The motor 39 is stopped, so that the crucible 80 isstationary. Then the rheostats 104 are each separately adjusted toobtain an even temperature from all segments 93, 94, 95, and 96, thisbeing dope by observing the silicon while cooling the crucible 80 to thesolidification point of silicon and making sure that icing occurs evenlyover the surface, adjusting the separate segments until it does.Observation is through the windows 46. The adjustment of the segments93, 94, 95, and 96 is then complete, and no further adjustment in therheostats 104 is made during that particular crystal unless randomsolidification is observed, in which instance readjustment is made.

In the third step, the seed rod 30, carrying a seed crystal is loweredinto the molten silicon. Before this, the motor 39 is again started torotate the crucible 80 slowly, while the motor 38 rotates the seed rod30 in the opposite direction at about the same speed. The temperature ofthe upper surface of the liquid has now to be adjusted, so that itsperiphery is above the melting point so that random solidification willnot occur at the periphery. The center of the upper surface is keptbelow the melting point and above the freezing or solidification pointof the silicon. To do this, the heaters and 51 are ini-tally separatelyadjusted, the heater 50 generally being at this time a few degreescooler than the heater 51. In other words, there is a temperaturedifferential. In the entire process, the bottom vand sides of thecrucible 80 have to be kept above the melting point of the silicon toprevent the growing crystal from adhering to any point on the crucible80.

With the temperature properly adjusted, the seed crystal is lowereduntil it just enters the liquid. Then it is raised, forming a meniscusand crystal growth begins. At this stage, the potentiometer 88 is turnedto lower the power to all segments of the heater 50 and to the heater51, to lower the temperature rapidly, while the rod 30 is pulled veryslowly, to get the maximum growth rate.

As the crystal approaches the desired diameter-a value slightly lessthan the inside diameter of the crucible 80 the pull rate on the rod 30is increased. The temperature is maintained, and the ingot shouldersoff.

Once the maximum diameter is achieved, the pull rate and thetemperatures of the heaters are varied to maintain the minimumvariations in that diameter. As the liquid level drops, thepotentiometer 88 is turned to lower gradually the heat to both heaters50 and 51, and also the temperature differential is lowered by adjust- 1ment of the heater 51 through the r-heos-tat 104. The

pulling continues in this manner. If freezing occurs at the cruciblewall, the peripheral heat is increased and the frozen portion remelted;if freezing occurs beneath the ingot (stopping rotation of the cruciblethe central heat is increased to remelt that frozen portion. The gradualdownward temperature adjustment continues with or without theseinterruptions until all the silicon is gone. The'power then is turnedoff, the pulling stopped and, after cooling, the crystal ingot iswithdrawn.

The present invention makes it possible to grow different kinds ofcrystals and to make adjustments in each.

kind of crystal. A typical ingot of the type just discussed has asloping upper end, but a substantially flat upper end can be provided bytaking care in the fourth step to have an initialpull rate that isnearly zero to get the maximum radial growth with minimum verticalgrowth. Then, when the silicon reaches its maximum desired diameter,both the pull rate and the temperature are increased to shoulder off thecrystal. Then the pull rate is kept substantially uniform and thetemperature gradually decreased so as to hold this diameter, and pullingis continued until all the silicon is exhausted.

By varying the type of seed and the shape of the crucible, other shapescan also be made in a manner now quite apparent from the foregoingexplanations.

7 c To those skilled in the art to which this invention relates, manychanges in construction and widely differing embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

I claim:

1. An electrical-resistance heater comprising a unitary tube of graphitewith a radially inwardly extending flange at its lower end terminatingin an inner periphery, said tube having a series of first evenly spacedaxially extending slots extending through said tube from the lower endup to a distance short of the upper end, leaving bridges there, and,midway in between said first slots, a series of second evenly spacedaxially extending slots extending through said tube from the upper enddown through said lower end except for bridges at the inner periphery ofsaid flange, so that there is a continuous graphite bar of great length.

2. The apparatus of claim 1 wherein said bar has a substantiallyconstant cross-sectional area at all points.

3. The apparatus of claim 2 wherein heater segments are provided byelectrical contact members evenly spaced around the lower end of saidtube.

4. An electrical-resistance type heater comprising a graphite ringhaving an inner periphery and an outer periphery with a first series ofevenly disposed radiallyextending notches extending from said innerperiphery through said graphite and out to outer peripheral bridges anda second series of evenly disposed radially extending notches, eachmidway between two notches of said first series, extending from saidouter periphery in to inner peripheral bridges, to make a continuousgraphite bar.

5. The apparatus of claim 4 wherein the cross'sectional area of said baris constant.

6. The apparatus of claim 5 having two contact members exactly 180apart.

References Cited in the file of this patent UNITED STATES PATENTS

